Ive always just used home made cut ones, using a reasonably thick plastic sheet, have never actually weighed them, but unlike Staeph, I prefer to use them to keep pokey stuff being in direct contact with the tent floor. Im pretty careful about clearing the pitching spot of rocks, twigs or whatever, but there have been times where circumstances dont give a lot of choice, ie cement or lots of gravel or whatever, and so its been nice to keep the tent floor directly out of contact with this stuff.
For a 1, 2 or 3 person tent, its not much extra weight to carry--but as Staeph says, it comes down to you deciding if its worth it (if I can avoid pinhole punctures that need to be patched, Ill take it, plus who wants wet stuff in a tent in the middle of the night in a huge downpour)